Thermoplastic vulcanizates (“TPVs”) comprise blends of dynamically cured elastomers and thermoplastic resins where the elastomer is dispersed within the thermoplastic resin phase as finely-divided elastomeric particles. TPVs advantageously demonstrate many of the properties of thermoset elastomers, yet are processable as thermoplastics.
Typically the elastomer used in forming the TPV is an ethylene, α-olefin, diene rubber copolymer, such as an ethylene, propylene, and diene monomer (“EPDM”) rubber. EPDMs produced using Ziegler-Natta catalysts have been favored for making TPVs due to their high molecular weight (as measured by their Mooney viscosity). However, this high Mooney viscosity, e.g., greater than 200 ML(1+4@ 125° C.), can result in difficulties when processing the rubber. As such, extender oils are often added to the polymer production process to reduce the apparent viscosity, and commercially available very high molecular weight EPDM rubbers typically contain from about 50 to about 125 phr of extender oil. While this aids in the extrusion of the EPDM, the oil-extended EPDMs are typically provided in bale form. As such an additional step is needed in the TPV production to granulate and partition the rubber. This increases the operation complexity and can require an addition of a large amount of extender filler and/or partitioning agents, such as clay. However, partitioning agents, such as a clay, are high density materials, and therefore a large addition of clay results in an increase of the density of the TPV.
EPDM rubbers have been produced using metallocene catalyst systems, for example as described in WO 2000/26296. Metallocene EPDMs can be advantageously supplied in pellet form and can contain little to no extender oil. However, when low- or non-oil extended EPDM rubbers are used to make TPVs, a large amount of process oil typically needs to be added into the extruder to ensure proper mixing of the TPV ingredients. However, large amounts of process oil can be difficult to consistently mix and incorporate into the thermoplastic vulcanizate which can lead to compounding process instability and undesirable TPV properties.
U.S. Patent Application No. 2008/0033089 describes a TPV composition comprising a dynamically-cured rubber; from about 20 to about 300 phr thermoplastic resin and from about 30 to about 250 phr additional oil. The rubber is a multimodal composition comprising 10-50 phr of extender oil, and 45-75 wt % of a first polymer fraction having a Mooney viscosity greater than 150 ML (1+4@ 125° C.) and 25 to 55 wt % of a second polymer fraction having a Mooney viscosity of 20-120 ML, where the polymer fractions have been polymerizing using a Ziegler-Natta catalyst system. The Examples in US 2008/0033089 were conducted in Brabender batch mixers.
US Patent Application Publication No. 2011/0028637 describes methods for preparing a TPV, using an elastomer that is not prepared by gas-phase polymerization methods and which has less than 75 phr oil. The method includes introducing a curative to the extruder after introducing the elastomer, introducing oil to the extruder after introducing the elastomer but before or together with introducing the curative, and introducing additional oil to the extruder after introducing the curative.
U.S. Patent Application Publication No. US 2011/0112238 describes a process for preparing a TPV using an olefinic copolymer rubber that is characterized by a multimodal molecular weight, an average branching index of greater than 0.8, and that includes less than 10 phr oil and less than 1 phr non-rubber particulate, and that is in the form of granules having a particle size less than 8 mm. The rubber is introduced to the extrusion reactor contemporaneously or sequentially with a thermoplastic resin, an oil, and a cure system.
U.S. Patent Application Publication No. 2013/0316111 describes a composition comprising: (A) a first composition comprising: i) a first interpolymer comprising, in polymerized form, ethylene, an alpha-olefin, and a nonconjugated polyene; ii) a second interpolymer comprising, in polymerized form, ethylene, an alpha-olefin and a nonconjugated polyene; and wherein the first composition has an MWD less than, or equal to, 3.5, a Mooney Viscosity (ML (1+4 @125° C.)) greater than, or equal to, 80, and an [(ML(1+4 @125° C.))/Mw(conv)]*1000 greater than 0.429 mole/g; (B) a thermoplastic polymer; and (C) a vulcanization agent.
There remains a need for methods of producing TPVs that have good physical properties using elastomers that have little to no oil extension that are provided in pellet or granule form. For example, there is a desire to provide TPVs that exhibit good physical properties, such as low extrusion surface roughness, that are easily processable.